1. Field of the Invention
The invention relates to an organic electroluminescent device (which is referred to hereinafter as an “organic EL device” in some cases) and, in particular, relates to an organic EL device which obtains light emission of high luminance with low driving power.
2. Description of the Related Art
Organic electroluminescent devices containing a thin film material that emits light by excitation due to application of electric current are known. The organic electroluminescent devices obtain high-brightness light emission at low voltage and therefore have broad potential applications in fields such as cellular phone displays, personal digital assistants (PDA), computer displays, car information displays, TV monitors, and general illumination, and they also have advantages of reducing the thickness, weight, size, and power consumption of the devices in the respective fields. Accordingly, such devices are greatly expected to become the leading devices in the future electronic display market. However, there are still many technical problems to overcome, such as with respect to luminescence brightness and color tone, durability under various ambient operating conditions, and mass productivity at low cost, in order for these devices to be practically used in these fields in place of conventional display devices.
Particularly, one problem is to improve luminance and driving power. First of all, it is a problem to realize high luminance in many of the above-described devices, when small thickness, light weight and small size thereof are aimed for. When the small thickness and the light weight are aimed for, it is demanded to realize compactness and light weight of not only the devices but also the driving power sources thereof. Particularly, when the power is supplied from a primary or secondary cell, power saving is a major problem, and it is strongly desired to obtain a high luminance with a low driving voltage. Conventionally, in order to obtain a high luminance, a high voltage has been required, and as a result, power is quickly consumed. Further, the high luminance and the high voltage result in deteriorating durability of the devices.
In JP-A Nos. 4-297076 and 2000-196140, attempts in which an organic compound layer which is doped with a metal compound dopant as an acceptor is arranged in a hole transport layer are disclosed. However, introduction of the acceptor in the hole transport layer has a disadvantage of deteriorating light emitting efficiency. Therefore, a measure for improving driving durability without deteriorating light emitting efficiency is demanded.
Further, in WO 2005-064994A1, it is disclosed to continuously change an acceptor concentration such that it becomes higher in a portion which is nearer to the light emitting layer. However, in this case, the effect of the acceptor is not sufficiently exerted, and sufficient improvement can not be obtained in each of light emitting efficiency and driving durability.